1. Field of the Invention
This invention relates to a safety dyna-bender, which has a center cord matched with a pair of cup-shape positioning anchors installed therein for protection against any mishaps unexpectedly occurring during bending exercise.
2. Description of the Prior Art
The structure of the conventional dyna-bender is shown in FIG. 1, which includes a steel spring coil 10 as the main component, a pair of metal handle pipes 11 and 12 separately connected to the steel spring coil 10 at both ends, a pair of hand grips 13 and 14 rigidly coupled with the handle pipes, and a pair of wrist rings 131 and 141 respectively provided at each end of the hand grips 13 and 14. The defects of the conventional dyna-bender are as follows:
(1) As the steel spring coil 10 is manufactured in mass production, minor flaws can easily be incurred in the operations of drawing and heat treatment. As a result, the internal stress and the crystal structure of the steel spring coil 10 are not formed in complete normal condition and easily subject to the harmful effect of stress concentration and over hardness. Henceforth, the steel spring coil 10 will soon become fragile and easily be broken during bending exercise (as shown in FIG. 2).
(2) The steel spring coil 10 is susceptible to external damage such as scraches, indentations, etc., caused by the machine tools during manufacturing process and handling operations, and its metal crystallization is often badly affected therefrom. Consequently, when the internal stress is concentrated at the damaged spot so as to have the metal crystal seriously impaired thereat, the steel spring coil 10 will also be broken once the application of the external force is beyond its load limit.
(3) The way of connection between the steel spring coil 10 and the handle pipes 11 and 12 is usually done by force-fully thrusting the end of the handle pipes into the open recess of the steel spring coil 10 at both ends thereof. Because of the difference of the internal diameter R of the steel spring coil 10 effected during manufacturing process, the joints formed thereof may be either too tight or in a slack condition therebetween. In such kind of states, if the connection is too tight, the jointed portion of the handle pipes 11 and 12 within the steel spring coil 10 will not be sufficiently engaged, or the contacting area thereof be a small one, or the contact thereof be even made only on some projected points by the handle pipes 11 and 12 along the inner surface of the steel spring coil 10. Therefore, the connection made between the handle pipes 11, 12 and the steel spring coil 10 is not perfect. When the steel spring coil 10 is bent in the way as shown in FIG. 3A during exercise, a friction force F.sub.1 in the direction as shown by the arrow will occur at side A, while another friction force F.sub.2 in the reverse direction will also take place at side B. Referring to FIG. 3B, when the applied force is reduced or completely ceased thereat, the steel spring coil 10 will recover, and another friction force F.sub.1 ' and .sub.2 ' at both sides thereof as shown by the arrows will also occur in the reverse direction to the time before as in FIG. 3A. During exercise, the repeated bending and recovering operations of the dyna-bender will naturally cause gradual wearing-away of the binding area between the steel spring coil 10 and the handle pipes 11 and 12 through the frictions mentioned above. Under such circumstance, the steel spring coil 10 may suddenly break loose with the handle pipe 11 at one end as shown in FIG. 4, imposing a very dangerous condition thereabout.
(4) As shown in FIG. 1, a pair of wrist rings 131 and 141 are respectively provided at each end of the hand grips 13 and 14 for being put around the wrist of the user during exercise in order to prevent the hands of the user from accidental fail to grasp the hand grip and cause serious condition thereat. However, due to the fact that the wrist rings 131 and 141 are movably attached thereto by tying up the ends in a knot 132, 142 therein, the wrist rings 131 and 141 are easily retracted into the hollow section of the hand grips 13 and 14 and difficult to be drawn out therefrom. Therefore, without precausionary meansures, the user often neglect the existence of the wrist rings 131 and 141 and does not use them during exercise.